Thermionic valve circuits



, Aug. 81, 19 E. F. GOODENOUGH THERMIONIC VALVE CIRCUITS:

Filed March 29, 1935 INVENTOR ERNEST FREDEIC GOODENOUGH ATTORNEY Patented Aug. 8, 1939 UNITEE STATES PA'EENT fii i i THERMIONIC VALVE CIRCUITS Application March 29, 1935, Serial No. 13,700 In Great Britain March 29, 19 34 10 (llaims.

V This invention relates to thermionic valve circuit arrangements and more specifically to thermionic valve circuit arrangements of the kind wherein control voltages are amplified in a thermionic valve amplifier and then supplied to a device which is predominantly of a capacitative nature and is employed to translate electrical energy variations into variations of some other form of energy. Though not exclusively limited thereto the invention is primarily intended for use in apparatus wherein the translating device is a predominantly capacitative device adapted to translate electrical energy variations into light variations. Such devices are commonly employed in television receivers, a typical example of such a device being the socalled Kerr cell. As above stated the invention is not exclusively limited to its application to, circuits incorporating predominantly capacitative electro-optical translating devices for it is also applicable to arrangements incorporating predominantly capacitative electroacoustic translating devices, e. g., so-called electro-static loudspeakers.

Devices of the nature of the Kerr cell or of the nature of the electrostatic loudspeaker generally require relatively large voltage variations to operate them, and in practice this means that, if an economical apparatus is to be employed, a driving valve of large amplification and hence of rela-,

tively high impedance must be utilized. Practical requirements, of course, involve that there shall be a satisfactory output in terms of variations of translated energy (e. g., light variations or sound variations) at quite high input frequencies; thus in the case of an electrostatic loudspeaker it is highly desirable that a sufficientand as nearly as possible proportional response shall be attained at high acoustic frequencies as well as at low while similarly in the case of a Kerr cell or the like, it is highly desirable that satisfactory light variations shall occur in response to television 7 or like signals of high as well as of low frequency. These requirementslead to difficulties in practice, and the present invention is concerned with, meeting these difficulties.

For the sake of brevity in description it will be.

assumed for the remainder of this specification that the light translating device in question is a Kerr cell, although as above stated the invention is applicable to circuits incorporating other predominantly capacitative electrical energy translating devices. a

A more detailed description of the invention follows and is illustrated in the accompanying drawing in which,

Fig. 1 shows the conventional method of connecting a Kerr cell to a thermionic amplifier;

Fig. 2 shows the theoretical equivalent of the anode circuit of Fig. 1;

Fig. 3 shows one embodiment of the invention, comprising a thermionic amplifier with an odd number of stages and a Kerr cell;

Fig. 4 shows one embodiment of the invention comprising a thermionic amplifier with an even number of stages and a Kerr cell;

Fig. 5 shows an alternative arrangement of Fig. 3;

Fig. 6 shows a modification of connection of the Kerr cell in which the cell is isolated iron the anode circuit of a thermionic valve; and

Fig. '7 shows a modification of the embodiment of the invention for an odd number of stages in a thermionic amplifier acting in conjunction with a Kerr cell.

A common method of connecting a Kerr cell which is to be operated by received television or like signals, is illustrated diagrammatically the accompanying Fig. 1 in which the said signals are applied to a pair of input terminals marked IN, one of which is usually earthed and the other of which is connected, for example through a coupling capacity C to the grid G of an amplifying valve V, e. g., a triode, having a resistance R between its anode and a source of anode potential (not shown but connected at +HT) the Kerr cell K being connected at one end to the anode and at the other end to the earthed input terminals and also through a capacity shunted resistance combination C1 GL to the cathode F, an input resistance IR being connected between the grid G and said earthed input terminal. The theoretical equivalent of the anode circuit of Fig. 1 is shown in the accompanying Fig, 2 where Z represents the internal resistance of the valve V, C the capacity of the Kerr cell and r the resistance value of R. In Fig. 1 e represents the input voltage and in Fig. 2 e is the product of the input voltage 6 and the magnification factor ,u of the valve V. As will be appreciated since the Kerr cell is from the electrical point of view practically a condenser, there will be an attenuation effect which may well become serious at high frequencies. This attenuation results from the fact that the anode circuit comprises an internal resistance so that the voltage available at the output terminals is a function of the current flowing to the load circuit. This follows from the well known'fact that the voltage occurring in the plate circuit of an amplifier tube may be considered a generator having a voltage equal to the grid incremental voltage multiplied by the amplification constant of the tube connected in series with two resistances, one representative of the internal impedance of the tube, and the second representing the impedance of the useful load circuit. It will be appreciated that under these circumstances the voltage measured across the terminals of the usual load circuit is equal to the equivalent generator voltage minus the internal impedance of the tube multiplied by the serial current. As this current increases, the voltage across the useful load circuit becomes less and less. In the case here under consideration, as the frequency of the voltages applied to the Kerr cell are increased, since the reactance of the cell decreases with frequency, more and more current tends to flow through the cell as the frequency is increased, and consequently, less voltage is available to produce the double refractive effect. The Kerr cell, as it will be appreciated, is dependent not so much upon the current that flows through it as it applies to the voltage across it. It is in this sense that the terms attenuation effect or frequency distortion are used. This attenuation effect can, of course, be reduced by making the resistance value r small in relation to the anode-cathode resistance Z but this in turn involves a reduction of the output voltage available at the terminals of the Kerr cell.

In the circuit arrangement of Fig. l the voltage across the Kerr cell may be expressed by the equation v dv v;ieZ;CZ (1) where v is the instantaneous voltage across the cell, c is the input voltage to the valve, .1 is the amplification factor of the valve, Z the anodecathode impedance thereof, and r the resistance in series between the anode thereof and the anode potential source and C the capacity of the Kerr cell.

In Equation (1) is the instantaneous signal current flowing in the resistance r and is the current flowing in the condenser due to a change of e and hence of o. It will be apparent therefore that undesired attenuation effects will occur by reason of the term in Equation 1) and the present invention consists in adding to the signal or control voltage an in-phase component proportional to the quantity and of such magnitude as to make the term containing dv s;

equal or approximately equal to zero. In other words, if the valve is capable of delivering the current dv at the attenuation which would arise due to the delivery of this current is, according to this invention, compensated for by the added in-phase component. Probably the most convenient way to obtain this voltage is to connect a resistance of suitable value in series with the Kerr cell or other capacitative device in question. The capacity current flowing through this resistance will set up a voltage drop, the whole or part of which may be added to the normal signal input voltage to the valve.

One way of carrying out this invention is illustrated diagrammatically in the accompanying Fig. 3 which shows an embodiment wherein a plurality of valve stages V1 V2 V3 in cascade are utilized. The cell K is connected between the anode of the last valve V3 and a suitably chosen point T1 in the cathode lead circuit of any odd numbered preceding valve stage (e. g., as shown the valve V1) the last valve stage V3 being counted as No. 1 the next preceding stage V2 being counted as No. 2 and the next preceding stage V1 as No. 3 and so on if there be any more stages.

In Fig. 3 the Kerr cell K is connected between the anode of stage I (V3) and a tapping point upon the resistance GL1 which is between the cathode of stage 3 (V1) and the earthed input terminal, the said tapping point being connected to the cathode of stage 3 (V1) through a condenser C1. A similar method of connection may be used irrespective of the number of valves in cascade but in order that the necessary in-phase superimposition may be obtained the Kerr cell terminals remote from the anode of stage I must be connected to an odd numbered preceding stage.

It will be appreciated that in essence the circuit of Fig. 3 may be regarded as a modification of Fig. l, the arrangement of Fig. 1 being modified by connecting that electrode of the Kerr cell K which is remote from the anode of the valve to whose anode it is connected to an adjustable tapping point T1 upon the resistance GL1 which is connected between the cathode of an odd numbered preceding valve (the valve V1 of Fig. 3) and earth the condenser C1 being now shunted between the cathode and the tapping point T1. The tapping point T1 is so adjusted as to result in the superimposition upon the signal or control potentials proper, of an in-phase component of sufiicient magnitude to produce the desired compensation result. The point to which the tapping should be adjusted for complete compensation will be understood from the following description:

Let T1 be the value of the resistance between the tapping point T1 and the earthed terminal. Then the voltage '0 available at the terminals of the Kerr cell will substantially be as given by the equation where 91 is the gain of the valve stages V1 and V2, .11. is the amplification factor of the valve V3 and e is the voltage applied across the terminals IN.

For complete compensation, i. e., for o to contain no terms involving Of course, if ,ugm is made greater than Z+r1 the circuit will oscillate. In practice the tapping point is made variable so as to'facilitate adjustment to the theoretically required value.

In a modification illustrated in the, accompanying Fig. 4 and suitable for use in cases where the amplifier energizing the Kerr cell consists of at least two stages in cascade, the Kerr cell is connected between the anode of stage I (V2 of Fig. 4) and a tapping point upon the input resistance 131 in the grid circuit of stage 2 (V1 of Fig. 4) or any other preceding even numbered stage (if there be no more). For example in the case of Fig. 4 where there are only two stages in cascade, the Kerr cell is shown connected between the anode of stage I (V2) and a tapping point T1 upon the input resistance 1R1 of stage 2 (V1). For this type connection, if 1' be the amount of resistance in effective series with the grid leak input resistance of the even numbered stage to which connection from the cell is made, the following equation substantially gives the value of the voltage across the Kerr cell:

where ,u' is the total gain of the amplifier counting from and including the valve in whose grid circuit r is connected, 6 is the voltage across the terminals IN and V is the resistance in series with the valve V2 and or is the attenuation due to the network comprising the grid leak (input) resistance shunted by the anode circuit of the previous valve in parallel with its anode resistance. That is to say, for complete compensation In a yet further modification shown in the accompanying Fig. 5 the Kerr cell K is connected between the anode of stage I (V3) and a suitably chosen tapping point T2 in a resistance R1 in series with the anode of any odd numbered preceding stage. Thus, as shown, for the case of a three valve amplifier, the Kerr cell may be connected between the anode of stage I (V3) and a tapping point upon the resistance R1 connected between the anode of stage 3 (V1) and the anode potential source therefor. With this arrangement Equation (6) above applies if r be taken as the amount of resistance employed for feed back purposes in the plate circuit of the odd numbered stage, 11' being taken as the gain of the amplifier counting from and including the valve following that in whose anode circuit 1' is included and a being taken as the attenuation of the anode circuit network.

In all the embodiments of the invention above described the Kerr cell is polarized by voltage drop occurring either across a valve or across an anode resistance. This is not, of course, a necessary condition and the Kerr cell may readily be isolated from the valve amplifier so far as direct current is concerned, by interposing a suitable blocking condenser of high value, and in such a case any desired value of polarizing potential obtained from any convenient independent source, may be employed. This expedient of isolating the Kerr cell as regards direct current may be adopted for any of the embodiments above described; forexample, as shown in the accompanying Fig. 6 any of the said embodiments may be modified by inserting in the connection between one terminal of the Kerr cell and the anode of a valve, a condenser BK of high value.

In the modification shown in Fig. '7 voltage developed across a resistance AB in series with the Kerr cell K is applied to'the grid of an ad ditional value V4 which acts as an ordinary amplifier and is additional to the valves preceding the Kerr cell. The output from V4 is superimposed upon the grid of the valve V3 in whose plate circuit the cell K is connected. Since the voltage set up across AR is amplified by an amplifier having an odd number of stages (in Fig. 7 only one stage) the superimposition may be effected upon the grid of any odd numbered valve in the cascade preceding the cell. If said amplifier had an even number of stages the superimposition would be effected upon the grid of any even numbered valve in the cascade preceding the cell.

Having now described the invention, what is claimed and desired to secure by Letters Patent is the following:

1. In an amplifying system, wherein is provided an output circuit including an energy translating device of capacitative reactance, the method of correcting for frequency distortion which comprises the steps of developing-electrical signal energy, amplifying the developed energy, supplying the amplified energy to the output circuit, deriving energy proportional to the rate of change in voltage across the translating device, and amplifying the derived energy in phase with the developed energy.

2. In an amplifying system, wherein is provided an output circuit including an energy translating device of capacitive reactance, the method of correcting for frequency distortion which comprises the steps of developing electrical signal energy, amplifying the developed energy, supplying the amplified energy to the output circuit, deriving energy proportional to the product of the capacity of the translating device and the rate of change in voltage across the translating device, and amplifying the derived energy in phase with the developed energy.

3. A thermionic amplifying arrangement, comprising an amplifier, an input circuit for said amplifier, an output circuit for saidamplifier, a load circuit comprising an energy translating device of substantially capacitative reactance, means connecting said output circuit to said load circuit, and means for returning a portion of the output energy of said amplifier to the input of said amplifier in accordance with the rate of change of voltage across the translating device.

4. A thermionic amplifying arrangement, comprising an amplifier, an input circuit for said amplifier, an output circuit for said amplifier, a load circuit comprising an energy translating device of substantially capacitative reactance, means connecting said output circuit to said load circuit, and means for returning a portion of the output energy of said amplifier to the input of said amplifier proportional to the product of capacity of the translating device and the rate of change of voltage across the translating device.

5. A multi-stage thermionic amplifier, an output circuit for said amplifier, a load circuit for said amplifier, said load circuit comprising a capacitative translating element and means for compensating for distortion across the translating device, said compensating means comprising a resistance connected in series with the translating device and constituting a part of the grid cathode circuit of one of the preceding stages of said amplifier, and means connecting the output circuit to the load circuit.

6. A multi-stage thermionic amplifier, an output circuit for said amplifier, a load circuit for said amplifier, said load circuit comprising a capacitative translating element and means for compensating for distortion across the translating device, said compensating means comprising a resistance connected in series with the translating device and constituting a part of the anode cathode circuit of one of the preceding stages of said amplifier, and means connecting the output circuit to the load circuit.

7. A mu1ti-stage thermionic amplifier, coniprising an output circuit, a load circuit connected to said output circuit, a capacitative translating device and compensating means connected in said load circuit, said compensating means comprising a resistance connected in series with the translating device and the input of one of the preceding stages of the amplifier.

8. An arrangement as claimed in claim 7 wherein the capacitative translating device is an electrostatic loud speaker.

9. An arrangement as claimed in claim 7 and wherein the capacitative translating device is a Kerr cell.

10. In an electrical translating system comprising a thermionic amplifier with an input and an output circuit, and an electrical capacitative translating device connected to said output circuit, the method of compensating for frequency distortion comprising impressing signal energy upon the input circuit, deriving a portion of the output energy actuating the device of predetermined magnitude, and impressing the derived portion .upon said input circuit in phase with the signal impulses.

ERNEST FREDERICK GOODENOUGH. 

